Chemistry Reference
In-Depth Information
CCRF plasmas between two circular plane electrodes made of stainless steel
(10cm in diameter, 2.5cm distance) were studied in a cylindrical vacuum chamber
(300mm in diameter, 500mm high) by phase-resolved optical emission spectroscopy
(PROES) [119,144]. The plasmas were ignited in pure oxygen at pressures rang-
ing from 20 to 100Pa. Only one of the electrodes was powered at a frequency of
13.56MHz (rf cycle of 74ns), whereas the other electrode and the walls of the vacuum
chamber were grounded. Thereby, a negative self-bias voltage from
550V
developed at the powered electrode corresponding to rf powers from 10 to 100W.
Despite the wide application of capacitive rf discharges in molecular gases,
combining the physics of nonequilibrium nonstationary plasmas with the complexity
of reactive plasma processes, a complete description of such systems is still missing.
The phase-resolved optical emission spectroscopy provides temporal resolution
to investigate the behavior of the plasma-induced optical emission within the RF
cycle and therefore the dynamics of energetic electrons and other species [120,121].
The particle-in-cell (PIC) method combined with Monte-Carlo collisions (MCC)
allows an accurate resolution of charged particles distribution functions and their
fluxes to the electrode [81,122-124]. The model resolves one spatial but three velocity
components (1d3v) and is a reduced model because out of up to 75 listed reaction and
scattering processes [125], which potentially affect the properties of the discharge,
only 20 are included in the model. These 20 are the ones with the largest cross sections
(see Table 9.2), representing the most important atomic and molecular processes in
the oxygen plasma.
The cross-section database for collisions in oxygen plasmas was critically
assessed [123] and corrected cross sections for elastic scattering (O
−
,O
2
), recombi-
nation (O
2
,O
−
), associative detachment (O
−
,O
2
), and charge exchange (O
2
,O
2
),
were implemented.
Although the simulated system is actually 3D, the discharge behavior is assumed
to be 1D along the axis. This is supported by the fact that the electrode diameter
is larger than the electrode spacing of
L
−
70 to
−
4.5 cm used in the simulation and that
the radial dependence of the plasma parameters close to the symmetry axis can
be neglected. Disregarding the electric asymmetry between the powered and the
grounded electrode, this part of the discharge can be simulated by a planar, 1D
model. This planar model retains only one spatial coordinate
X
, with 0
=
≤
X
≤
L
,
where
L
is the distance between the electrodes.
In the simulations, the initial electron density and temperature were chosen as
n
e
0
=
8
×
10
9
cm
−
3
and
T
e
0
=
10eV, respectively. The computational domain length
was
X
max
=
X
max
,
the absorbing wall boundary conditions were applied. Secondary electron emission
was neglected. The potential at
X
172 λ
D
0
=
4.5cm. At the positions of the electrodes
X
=
0 and
X
=
0 was fixed at 0 V, corresponding to the grounded
electrode. At the position of the powered electrode at
X
=
=
X
max
, the potential was
assumed to oscillate according to the applied rf voltage: φ
(
X
max
,
t
)
=
U
RF
sin
(
ω
RF
t
)
with ω
RF
/
13.56 MHz and a rf voltage amplitude
U
RF
ranging from 75 to 1000 V.
As the neutral gas density is much higher than the densities of charged species, the
neutral gas was treated as a background with fixed density and temperature. The
gas temperature was
T
n
2π
=
300 K, and pressure was varied between 20 and 60 Pa.
Only the dynamics of three charged particles (electrons, O
2
, and O
−
ions) were taken
=
